Chapter 13 Flashcards
A nucleus with an _______ atomic number or mass number has a nuclear spin that can be seen by NMR
odd
1H and 13C have two spin states called _______ and ________
alpha and beta
in the absence of an external magnetic field. spins of the magnetic nuclei are oriented _______ and are all _______ in energy
randomly
equal
when nuclei are placed in a external magnetic field (B0), the nucleus can align with the magnetic field in a way that complements (_____) or opposes (_____) that external magnetic field
alpha
beta
Nuclei oriented in the same direction as the external magnetic field are slightly _______ in energy than those that oppose the external magnetic field.
lower
alpha lower than beta
When are magnetic nuclei said to be in resonance?
when radio frequency energy is absorbed and the lower energy spin state flips to the higher energy spin state (alpha to beta)
In 1H NMR spectrum, NMR can only see _______
hydrogens
an NMR spectrum has the _______ region at the left (10) and the _______ region to the right (0)
downfield
upfield
What is the solvent used to dissolve an organic sample in NMR?
CDCl3
deuterium is invisible in NMR
What are the 3 basics for interpreting NMR?
1) chemical shift
2) integration
3) splitting
The chemical shift or position of an NMR absorption will depend on the density of the H atom: affected by
1)
2)
3)
1) inductive effect
- EN of atoms
2) hybridization
- pi effects
3) H-bonding
What is the formula for ppm?
observed peak (Hz) over operating frequency (MHz)
If a H is closer to more electronegative groups = _______ = _______
deshielded = downfield
Around what range do H connected to aromatic rings show up? (ppm)
7-8 ppm
Around what range do H connected to carbonyls show up? (O=C-CH3) (ppm)
2-3 ppm
around what range do H connected to O show up? (O-CH3) (ppm)
3-4 ppm
EN effects are _______, meaning more EN atoms will make the chemical shift more downfield
additive
Chemical shifts are affected by the _______ from an electronegative atom
distance
explain the pi effect
pi electrons move more easily than electrons in a sigma bond
Explain how the pi effect differs between double bonds and triple bonds
db pi e’s cause a downfield shift while tb pi e’s cause an upfield shift
H-bonding often reveals _______ signals because of constant breaking and reforming
broad
Different types of _____ = # of signals that will appear in 1H NMR
H
- draw lines of symmetry and count different H’s
Enantiotopic: H’s on carbon atoms are electronically equivalent but chemically distinct. These H will likely show _______ peak(s) in the 1H NMR
a single
Diastereotopic: H’s on carbon atom are neither chemically or electronically equivalent. These H are likely to show _______ peak(s) in the 1H NMR
2 different
- even though they are on the same carbon
What is the difference between enantiotopic and diastereomeric, and how are you supposed to tell the difference?
enenatiotopic = 1 peak
diastereotopic = 2 peaks
diastereotopic will have a chiral center on OG molecule
Integration: the relative _______ of the integral is proportional to the # of _______ contributing to the peak
height
H’s (hydrogens)
it is important to keep in mind that integration values are relative and that they represent the _______ of protons not absolute values
ratio
Multiplicity or spin-spin splitting results when _______ types of _______ are close enough that their magnetic fields influence each other
2 different
protons
- need to be H’s on neighboring Carbon
N+1 rule: # of peaks a H produces = (# of _______ H) +1
neighboring
What is the exception to the N+1 rule?
OH and NH ignore neighbors
Most spin-spin splitting is between protons on _______ carbon atoms. (equivalent protons do not split each other)
adjacent
Splitting tree diagrams:
What is the ratio for a doublet (d)?
1:1
Splitting tree diagrams:
What is the ratio for a triplet (t)?
1:2:1
Splitting tree diagrams:
What is the ratio for a quartet (q)?
1:3:3:1
Protons that split each other are said to be _______
coupled
The distance between adjacent lines of a splitting pattern is called the coupling constant (___) measured in Hz
J
Coupled protons will have the same value of _____
J
Using J values is most useful to distinguish between possible _______ of a compound or _______
isomers
stereoisomers
- Distinguish between cis and trans bonds
_______ provides a reference singlet at 0ppm or delta
TMS
describe the common splitting pattern for ethyl groups
2H quartet to 3H triplet
describe the common splitting pattern for an isopropyl group
1H septet to a 6H doublet
describe the common splitting pattern of a PARA-substituted aromatic ring
leaning doublets
Leaning doublets is common splitting pattern of _______
PARA-substituted aromatic ring
_______ results from different types of H with the same chemical shift
multiplets
- overlapping signals
more complex patterns can arise when a signal is split by two or more _______ proton types
non-equivalent
- ex styrene doublet of doublets
The doublet of doublets is common splitting pattern of _______
monostubstitued alkene
If H are rapidly moving in 3D space, the NMR signal will appear at the position where the _______ H would be detected = _______ signal
average
single
Any process faster than ________ will be recorded as an average by NMR spectroscopy.
0.01 sec
What are 2 ways that NMR can show two non-equivalent types of protons that split each other?
1) low temperatures
2) add big group (tBu)
if H are lost/gained rapidly because ______ or _______ is present, no neighbor information is retained
acid or base
- this is why OH and NH ignore neighbors
What is the only situation where OH splitting will appear on an NMR?
ultrapure ethanol
In what situation will the 3.5 peak disappear?
when OD is used
Proton exchange an be used to identify OH and NH NMR signals by shaking the sample with _______. Any exchangeable protons are quickly replaced by deuterium atoms, which are invisible to NMR
D2O
for 13C NMR, most important is the _______ of the individual arbon resonances
chemical shift
Values for 13C NMR are about ____ times the chemical shift of 1H NMR
20
13C NMR shows _______ carbon types
all
DEPT-90 shows signals for ______ carbons only
C-H
DEPT-135 shows
Positive: _______
Negative: _______
postive: CH, CH3
negative: CH2
